The definition of a mainstay is something that is depended on, or something that is necessary.
Fruits and vegetables are an example of the mainstay of a healthy diet.
- a stay, or line, extending forward from the mainmast, supporting it and holding it in position
- a chief support
- A chief support: Agriculture is a mainstay of the economy.
- Nautical A stay running forward from the mainmast to the deck of a sailing vessel.